In order to efficiently encode both speech and music signals, a complex audio encoding system is found effective which is used to switch between an encoding scheme suitable for speech signal and an encoding scheme suitable for music signal.
Patent Literature 1 below describes such a complex audio encoding system. In the audio encoding system described in Patent Literature 1, each frame is added with information indicative of the type of an encoding scheme used for generation of a coded sequence for the frame.
The audio encoding in MPEG USAC (Unified Speech and Audio Coding) uses three encoding processes, i.e., FD (Modified AAC (Advanced Audio Coding)), TCX (transform coded excitation), and ACELP (Algebraic Code Excited Linear Prediction). In MPEG USAC, TCX and ACELP are collectively recognized as LPD. In MPEG USAC, each frame is added with 1-bit information to indicate whether FD or LPD was used. When LPD is used in MPEG USAC, each frame is added with 4-bit information to define a procedure of using a combination of TCX and ACELP.
Furthermore, AMR-WB+ (Extended Adaptive Multi-Rate Wideband) of Third Generation Partnership Project (3GPP) uses two encoding schemes, i.e., TCX and ACELP. In AMR-WB+, each frame is added with 2-bit information to discern use of TCX or ACELP.